Hydrostatic, mechanical, power-split transmissions are comprised of one continuously variable hydrostatic transmission having one adjustable volume and one constant volume unit, one summarizing transmission and one range-change transmission with several shiftable gears. The power introduced, via an input shaft splits, on one side to the hydrostatic transmission and, on the other, to an input shaft of the summarizing transmission. The rotational speeds and torques of the hydrostatic transmission and of the prime mover are jointly passed by the summarizing transmission. Different gears can be selected with the aid of gear clutches by the output shaft of the summarizing transmission being coupled via different spur gear steps with the output shaft. Compared with mechanical transmission, continuously variable transmissions have the advantage that it is possible to drive at optimal rotational speed of the motor with no traction interruption occurring when shifting.
DE 39 03 877 discloses a hydrostatic, mechanical, power-shift transmission consisting of a four-shaft gear, planetary transmission and parallel therewith a continuously variable hydrostatic transmission, the same as other gears. By shifting gear clutches, several gears are implemented in each of which the hydrostatic transmission effects a continuous adjustment of the ratios of the whole transmission. In case of synchronous rotational speeds, the change of gear is effected load-free and without traction interruption. By virtue of the structure of the gear clutches and of the control provided, it is possible to activate a new gear and subsequently deactivate the old gear without any jolt during the shift operation. The reverse gear is implemented by its own gearset and a corresponding clutch adjustment, which is to be actuated, for activating the reverse gear.
From WO 97/01049 is known a power-split transmission provided with a mechanical and hydrostatic power branch disposed in a transmission housing. Both branches are operated, via a common input shaft, and summarized in a coupling gear. The coupling gear has several planetary gearsets and clutches and is connected with on output shaft. The coupling gear is situated upon the input shaft. The last planetary gearset of the coupling gear can be connected with a clutch for forward and reverse gears, said clutches being connectable with the output shaft. The operation change from forward to reverse gear and vice versa is effected via the interplay of the clutches. When stopped at zero rotational speed, both clutches are closed. Therefore, depending on the travel direction desired, the corresponding clutch has to be actuated first.
In WO 99/15813 are shown different embodiments of a hydrostatic, mechanical, power-split transmission in which at least the summarizing transmission and the clutches or also the hydrostatic transmission are situated co-axially relative to each other (in-line design). To adapt the traction needed, a group transmission can be provided having, for example, one field and one road group. The change between these groups, the same as between forward and reverse gear, is implemented by actuating a corresponding clutch. The shift to the other respective range is preferably effected after a defined dwelling time on the corresponding ratio point or within a delimited ratio range, i.e., during stoppage or constant speed.
Those transmissions, however, have a basic problem in common that the hydrostatic transmission is connected with a gear transmission, therefore, they no longer constitute a really continuously variable transmission. The shifting of clutches are needed for changing to different ranges, especially when changing from forward to reverse gear and vice versa. The actuation of clutch devices is required, whereby a traction interruption and time delay result negatively impairing the traveling comfort.
Mainly in agricultural, commercial vehicles, the gear change operation between opposite travel directions acts negatively upon the driving properties, such as the efficiency degree when starting. At the starting point, high idle power can appear which can correspond to a multiplication of the input power. This negatively affects the dimensions and weight of the transmission.
The Applicant's DE 101 22 823 A1 discloses a power-split transmission having one hydrostatic part, one summarizing transmission and one range change transmission in which a purely hydrostatic transition driving range is provided for speeds between low reverse gear and low forward gear without a range transfer and without shifting of clutches, especially at zero speed and, attached thereto, a continuously variable hydrostatic, mechanical, speed range with power-split for high speeds. The hydrostatic transmission part, the summarizing transmission and the range change transmission are disposed co-axial with each other upon the intermediate shaft.
Thereby no range transfer is required when changing from low forward and reverse speeds so that in this speed range the shifting of clutches is eliminated, but the length has proved disadvantageous.
The problem on which this invention is based is, starting from said prior art, to provide a hydrostatic, mechanical, power-split transmission in which at zero speed, no clutches have to be shifted, a change of travel direction between forward and reverse is possible without delays, which has a good degree of efficiency and makes do with few speed ranges. Besides, the required length is to be considerably reduced compared to the prior art.